Botkin Environmental Science Earth as Living Planet 8th txtbk

98 CHAPTER 5 Ecosystems: Concepts and Fundamentals
The bog also fills in from the bottom as streams
carry fine particles of clay into it (Figure 5.17b, c). At
the shore, the floating mat and the bottom sediments
meet, forming a solid surface. But farther out, a “quaking
bog” occurs. You can walk on this mat; and if you
jump up and down, all the plants around you bounce
and shake, because the mat is really floating. Eventually,
as the bog fills in from the top and the bottom,
trees grow that can withstand wetter conditions—such
as northern cedar, black spruce, and balsam fir. The formerly
open-water bog becomes a wetland forest. If the
bog is farther south, it may eventually be dominated by
beech and maple, the same species that dominate the
late stages of the dunes.
Old-Field Succession
In the northeastern United States, a great deal of land
was cleared and farmed in the 18th and 19th centuries.
Today, much of this land has been abandoned for farming
and allowed to grow back to forest (Figure 5.18). The first
plants to enter the abandoned farmlands are small plants
adapted to the harsh and highly variable conditions of a
clearing—a wide range of temperatures and precipitation.
As these plants become established, other, larger plants enter.
Eventually, large trees grow, such as sugar maple, beech,
yellow birch, and white pine, forming a dense forest.
Since these three different habitats—one dry (the
dunes), one wet (the bog), and one in between (the old
field)—tend to develop into similar forests, early ecologists
believed that this late stage was in fact a steady-state
condition. They referred to it as the “climatic climax,”
meaning that it was the final, ultimate, and permanent
stage to which all land habitats would proceed if undisturbed
by people. Thus, the examples of succession were
among the major arguments in the early 20th century
that nature did in fact achieve a constant condition, a
steady state, and there actually was a balance of nature.
We know today that this is not true, a point we will
return to later.
Coral Reef Succession
Coral reefs (Figure 5.19) are formed in shallow
warm waters by corals, small marine animals that live in
colonies and are members of the phylum Coelenterata,
which also includes sea anemones and jellyfishes. Corals
have a whorl of tentacles surrounding the mouth,
and feed by catching prey, including planktonic algae,
as it passes by. The corals settle on a solid surface and
produce a hard polyp of calcium carbonate (in other
words, limestone). As old individuals die, this hard
material becomes the surface on which new individuals
establish themselves. In addition to the coelenterates,
other limestone-shell-forming organisms—algae,
corals, snails, urchins—live and die on the reef and are
glued together primarily by a kind of algae. 16 Eventually
a large and complex structure results involving many
other species, including autotrophs and heterotrophs,
creating one of the most species-diverse of all kinds of
ecosystems. Highly valued for this diversity, for production
of many edible fish, for the coral itself (used in
various handicrafts and arts), and for recreation, coral
reefs attract lots of attention.
Succession, in Sum
Even though the environments are very different,
these four examples of ecological succession—dune, bog,
old field, and coral reef—have common elements found
in most ecosystems:
1. An initial kind of autotroph (green plants in three
of the examples discussed here; algae and photosynthetic
bacteria in marine systems; algae and photosynthetic
bacteria, along with some green plants in some
freshwater and near-shore marine systems). These are
typically small in stature and specially adapted to the
unstable conditions of their environment.
FIGURE 5.18 Old-growth eastern deciduous forest.
FIGURE 5.19 Hawaiian coral reef.